If I remember right, the barrel life of a 7 mag is about half that of a .308.
The thing about barrel life is that what usually gets toasted in the barrel is the throat, not the rifling. Well, the rifling will go sooner or later, but the throat is what goes first. Depending on the profile of the barrel and whether the barrel is even worth the effort, you can have the barrel set back and a new chamber cut to get more life out of the barrel.
This is one of the reasons that I got the barrel profile that I chose. My barrel has a 1.25" diameter at the breech end and this diameter continues for 5" toward the muzzle. I can have my barrel set back quite a few times or until the rifling actually goes.
In any case, a 7 mag is going to need work much sooner than a .308 will. The extent of the work necessary will depend on a few variables.
What will be right for you will depend on how much you shoot. I've been averaging about 500-600 rounds of .308 a month, unless I get to make a trip to someplace fun. In that case, I could eat up 500-1000 rounds in a weekend.
You also have to keep in mind that although a 7 mag is likely better suited to shooting 1000 yards, you probably won't be doing that all of the time. The ideal thing would be to have more than one rifle.
Another thing about .308 is that it's just plain fun to shoot. It has enough recoil to give you that thrill, but not so much that it is painful to shoot.